1. Field of the Invention
This invention relates generally to soft X-ray lithography systems and is concerned more particularly with an efficient source of soft X-rays for photolithography.
2. Discussion of the Prior Art
In photolithography, a suitable radiation source generally is disposed for beaming radiation through transparent portions of a shadow mask and onto aligned portions of a photoresist film on a workpiece. Thus, in the manufacture of microelectronic devices, such as integrated circuit semiconductors, for example, ultraviolet light may be beamed through a pattern of transparencies in a mask to expose selected portions of a photoresist film on a surface of a semiconductor wafer. Subsequently, the wafer may be passed through a developer bath for removing either the exposed or the non-exposed portions of the photoresist film. As a result, the pattern of transparencies in the mask is replicated on the surface of the wafer to provide access means for integrating corresponding circuitry with the semiconductor material of the wafer.
With increased density of the integrated circuitry, portions of the pattern in the mask may require dimensions in the order of one micrometer or less. However, ultraviolet light is diffraction limited in this range. Consequently, an ultraviolet light source is not suitable for resolving portions of the pattern having dimensions less than one micrometer. Therefore, in order to avoid the effects of diffraction, soft X-ray lithography systems have been developed in the prior art to provide the higher resolution required for replication of mask patterns having dimensions less than one micrometer.
These soft X-ray photolitography systems of the prior art usually include an X-ray tube having an envelope wherein an electron emitting cathode is disposed to beam electrons onto a focal spot area of soft X-ray emissive target material, such as aluminum, for example. However, the resulting K alpha X-rays emitted by the aluminum target material have an energy value of less than fifteen hundred electron volts, which is barely sufficient for passing through an excessively thin window in the tube envelope and through the transparent portions of the shadow mask to activate the selected portions of the photoresist film. Furthermore, aluminum exhibits an undesirably high vapor pressure at relatively low operating temperatures, such as one thousand degrees Centigrade, for example. Consequently, the instantaneous temperature of the focal spot area is maintained at a considerably lower value by limiting the incident electron energy, which also limits the intensity of the soft X-rays emanating from the focal spot area. As a result, the exposure time required to activate the selected portions of the photoresist film is relatively long, such as about ten minutes, for example, and is unsuitable for the manufacture of microelectronic devices.
Therefore, it is advantageous and desirable to provide a soft X-ray lithography system with source means for producing a beam of soft X-rays having sufficient energy and intensity to expose selected portions of an X-ray sensitive film on a workpiece in a time interval suitable for production techniques.